Disc drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to the user. In general, a disc drive comprises a magnetic disc that is rotated by a spindle motor. The surface of the disc is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. Each of the data tracks extends generally circumferentially around the disc and can store data in the form of magnetic transitions within the radial extent of the track on the disc surface. Typically, each data track is divided into a number of data sectors that store fixed sized data blocks.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to transmit an electrical signal that causes a magnetic transition on the disc surface, to write data. The magnetic transducer includes a read/write gap that positions the active elements of the transducer at a position suitable for interaction with the magnetic transitions on the surface of the disc, as the disc rotates.
As known in the art, the magnetic transducer is mounted by the head to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disc to either read data from or write data to the preselected data track, as the disc rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disc surface due to fluid currents caused by rotation of the disc.
One part of the motors used in disc drives is the bearing. The basic fluid dynamic bearing involves parts which must move relative to one another, such as a shaft and a sleeve or a thrust plate and a housing, separated by a small gap filled with a viscous fluid such as oil. One characteristic of fluid dynamic bearings is stiffness, which is a measure of the amount of displacement per unit force applied to the rotating element. The force could be due to gravity, an imbalance in the rotating element, or some other source. Another characteristic of fluid dynamic bearings is drag, which refers to the torque required to rotate the rotating element at a rated speed.
One problem with the fluid dynamic bearing is that the stiffness of the bearing is very sensitive to changes in temperature. This is because the stiffness depends on the viscosity of the bearing fluid, and the viscosity of the bearing fluid typically changes with temperature. The bearing drag is proportional to viscosity, and also varies significantly with temperature. In certain applications, such as the use of the fluid dynamic bearing in a disc drive motor, the bearing must operate over a wide range of temperature. As a result, a fluid dynamic bearing with adequate stiffness at one temperature will have a stiffness which is too high at lower temperatures, and too low at higher temperatures, making the fluid dynamic bearing unsuitable for such an application. The same problem occurs with respect to drag.
Prior art fluid dynamic bearings have the problem that stiffness and/or drag are highly dependent on temperature, making conventional fluid dynamic bearings difficult to use for certain applications, such as use in disc drives, VCR helical scanners, gyros, and other rotating devices. There is a need for a fluid dynamic bearing with a reduced dependence of stiffness and/or drag on temperature.
Several prior attempts to solve the problem of temperature dependence of stiffness and/or drag in a fluid dynamic bearing make the different parts of the bearing from materials with different coefficients of thermal expansion. See e.g. Gustafson U.S. Pat. No. 5,664,889. The materials from which the bearing is made can be chosen such that the fluid gap decreases as the temperature increases. A bearing with a small gap has a higher stiffness and/or drag due to the change in the viscosity of the fluid with temperature.